"From the shelves of your stores, a would-be mass shooter can select a Bushmaster AR-15[style rifle] just like the one that robbed our clients of their children," said the letter, according to a spokesman for the law firm Koskoff, Koskoff & Bieder.

Twenty-six people, including 20 children, were killed at the school in Newtown, Connecticut, by a gunman armed with a Bushmaster rifle.

The letter listed other AR-15-style rifles used in mass shootings, including the Smith & Wesson semiautomatic rifle used to kill 17 people at a high school in Parkland, Florida, on February 14. .

Gun retailers have been under pressure to stop selling military-styleAR-15-style rifles. Dick's Sporting Goods CEO Edward Stack said on Wednesday that his company would stop selling "assault-style rifles."

In addition, Stack said that Dick's would no longer sell high capacity magazines and was raising the minimum age for gun purchases from 18 to 21. Walmart, which stopped selling military-style semiautomatic rifles in 2015, also moved up the age for gun and ammunition purchases to 21, as did outdoor gear maker L.L. Bean and the grocery chain Kroger, which sells guns through its Fred Meyer stores.

The Bushmaster is a semiautomatic rifle of similar design to the AR-15 rifles used by the U.S. military. These guns, which are referred to as assault rifles by gun control advocates and as modern sporting rifles by the gun industry, are widely available to civilians in the United States.

Brian Rafn, a gun industry analyst for Morgan Dempsey, estimates that there are 9 million to 17 million modern sporting rifles in the United States, out of an estimated "civilian gun arsenal" of 455 million guns.

AR-15-style rifles and other guns sold briskly during the Obama administration, because gun advocates were afraid of the possibility of stricter gun control laws. Those fears have dissipated since the election of President Trump, a Republican endorsed by the National Rifle Association. Gun sales have suffered as a result.

American Outdoor Brands, which makes Smith & Wesson guns, reported a 33% decline in sales and a 65% decline in earnings on Thursday. Other gun manufacturers like Sturm Ruger and Vista Outdoor also reported dismal earnings earlier this year.